Electronic devices use power to operate. Power converters are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power converter a high voltage alternating current (ac) input is converted to provide a well regulated direct current (dc) output through an energy transfer element. The switching power converter typically includes a controller that provides output regulation by sensing the output and controlling it in a closed loop.
A power converter controller typically receives power for its internal blocks from a supply terminal. In some instances, this supply terminal may be referred to as a bypass (BP) pin/terminal, or a VDD supply. In power converters that include magnetic isolation or transformers, such as flyback converters, the voltage to the bypass pin/terminal is provided during normal operation after startup through an extra auxiliary winding on the transformer core. Thus, an auxiliary winding is commonly referred to as a supply or bypass winding. The ac induced voltage on the auxiliary winding is typically rectified and filtered by a supply terminal capacitor coupled to the bypass pin/terminal and the auxiliary winding to generate a dc supply voltage on the bypass pin/terminal.
A number of simple DC power supplies regulate the voltage of electronic devices using either series or shunt regulators. Many apply a voltage reference using a shunt regulator such as a Zener diode or an avalanche breakdown diode, or voltage regulator tube. Each of these devices begins conducting at a specified voltage and will conduct as much current as required to hold its terminal voltage to that specified voltage by diverting (i.e., shunting) excess current from a non-ideal power source to ground, often through a relatively low-value resistor to dissipate the excess energy. In a number of applications and integrated circuits (ICs), there is a need to provide a power-efficient auxiliary supply shunt regulator that operates in tandem with a lower output voltage regulator operating from a different input voltage. In such cases, the shunt regulator can take over and limit the output voltage should an auxiliary supply be present in the system.
One of the problems with past designs that operate in this manner is the difficulty of providing a linear shunt regulator that is stable into a large external capacitive load while operating in a lower power mode, just prior to the shunt becoming fully active. Traditionally, there two techniques have been used: (1) a comparator-based “bang-bang” operation that senses if VBP is rising close to the desired shunt regulation voltage so as to turn on the shunt regulation switch; and (2) a linear operation using a standard operational amplifier (op-amp) where the op-amp is used to close the feedback look and regulate stably when required. A drawback of the first approach is increased supply voltage ripple due to inherent oscillation. A drawback of the second approach is that the core op-amp uses significantly more current to remain stable into a large capacitive load, even when the shunt regulator is idle.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.